There is an inkjet printing apparatus which forms images on recording media by discharging ink from nozzles. The inkjet printing apparatus discharges a liquid ink from the ends of the nozzles by pressurizing the ink.
However, if air is in this liquid ink when the ink is pressurized, pressurization characteristics of the ink change and accordingly the ink may be not discharged at a desired speed with a desired amount, and/or fixation characteristics of the ink droplets dropped on recording media change, so that a problem of image quality deterioration arises. Then, there has been a well-known technique of removing air in ink with a degassing device provided.
In addition, as the upper limit of an ink consumption speed has been increasing due to increase in speed and definition of image forming with an inkjet printing apparatus, efforts have been made to increase efficiency of a degassing device and to make degree of degassing uniform, the degree of degassing being suitable for the ink consumption speed. There is described in Patent Document 1 a technique of: repeating, a desired number of times, a process of sending ink flowing out of a sub-tank to a degassing device to degas the ink and returning the ink to the sub-tank, wherein the sub-tank is disposed in an ink flow passage from an ink tank to nozzles, and the degassing device is connected to the sub-tank in parallel through a three-way valve; and thereafter switching the three-way valve to supply the ink from the sub-tank to the nozzles. There is described in Patent Document 2 a configuration to send ink in a sub-tank to a degassing device multiple times to repeatedly degas the ink, wherein the degassing device is disposed in a circulation flow passage attached to the sub-tank. There is described in Patent Document 3 a technique for increasing efficiency of degassing with a return flow passage to make ink flow, multiple times, in an ink flow passage where a degassing device and a sub-tank are arranged in series.